Recently we have reported on the phospholipid and fatty acid composition of the rabbit iris muscle (A.A. Abdel-Latif and J. Smith, Exp. Eye Res., in press). We also showed that the rabbit iris can incorporate (14C) arachidonic acid both in vivo and in vitro, and most of the radioactivity was recovered in phosphatidyl choline, phosphatidylethanolamine and phosphatidyl inositol. We propose to prelabel irises with (14C) arachidonate, isolate the above phospholipids by means of silicic acid chromatography, then use them as substrates for the iris muscle lipases. Emphasis will be placed on investigating the properties and conditions of arachidonate release from these phospholipids and the subsequent conversion of this polyunsaturated fatty acid into prostaglandis. The properties of the enzymes involved in this release, such as phophatidyl inositol phosphokiesterase and phospholipase A2 will also be investigated.